Tankless waterheater, isolation and pressure valves. Why not connect at supply?

[lookslikeahog]

Everything I have seen has shown these pressure/isolation/clean out valves to be connected right to the inlet and outlet of the tankless heater. I am replacing my tankless, and replacing and reversing the elbows vertically on the house cold/hot inlets (current configuration below) and getting rid of cpvc. Why couldn't I just connect these new isolation valves right to the supply elbows then, and run a short supply lines (10" or so) up to the new heater? I don't really see much of a difference.

 

[Geofd]

unless the manufacturer asks for that specifically I would remove both elbows at the wall and start over I would install unions on both sides so it can be removed easy next time